system-image-update: update to head revision
system-image-update: update to head revision

Update System::Image::Update utility to head revision to avoid experiemental
warning spam in logs.

Signed-off-by: Jens Rehsack <69d49c504a9219a55b8be0f905e8fe3c32ffb57f@netbsd.org>

rdm-dev/meta-jens,rdm-dev/meta-jens,rdm-dev/meta-jens,rdm-dev/meta-jens